Leave it to Johnny Depp to re-ignite the controversy over what made the Mad Hatter mad. As reported by Pat Ryan in the New York Times (3/7/10), Johnny, who plays the Hatter in a new film (trailer), told a news conference that he thought the Hatter was mad because of mercury poisoning from hats, saying he's "this guy who literally is damaged goods." But while it's true that some hats contained mercury back in Lewis Carroll's day, the symptoms of mercury poisoning -- including "excessive shyness ... and a desire to remain unobserved and unobtrusive" hardly fit the Mad Hatter.

Another theory holds that the Hatter was inspired by a French phrase, "il raisonne comme une huitre," or "he reasons like an oyster." The idea is that "huitre" sounds like "hatter." Seems unlikely. Logicians, noting that Lewis Carroll "was well known as a mathematician, think the Mad Hatter is really the Mad Adder. They cite this Hatter quote: "If you knew Time as well as I do ... you wouldn't talk about wasting it. It's him." In the Hatter's defense, physicist Robert A. Millikan wrote that the Hatter wasn't mad "when he gave to Time changeable, undependable, capricious qualities which we assign to personality."

The Hatter is concerned with time "since the Queen of Hearts has accused him of being a time-murderer." This adds heft to the theory that the Mad Hatter was based on "a top-hat wearing inventor, Theophilus Carter," who, in 1851, exhibited an "Alarm-Clock Bed" that tipped "the sleeper out of bed at the correct time." But the best "Mad as a Hatter" explanation comes from Lewis Carroll himself. Why a Mad Hatter, not a Mad Tailor or Shoemaker? (viaduct, why not a goose?) Carroll wrote, "I'm very much afraid I didn't mean anything but nonsense! Still, you know, words mean more than we mean to express when we use them."